Direct detection of RNA mediated by reverse transcriptase lacking RNAse H function

ABSTRACT

Disclosed is a method of detecting RNA molecules of interest in which reverse transcription primers unique to the RNA molecule of interest are used for reverse transcribing the RNA with a reverse transcriptase lacking RNAse H function and the resulting RNA/DNA hybrid is detected with an antibody specific for RNA/DNA hybrids. This method can be used to detect the presence of one or many specific RNA molecules which may be present in a sample, including RNA from different organisms (such as viruses, bacteria, fungi, plants, and animals), or RNA indicative of an infection, a disease state, or predisposition to a disease in an animal. The specificity of detection is increased relative to current detection methods involving probe hybridization since the reverse transcription primers are shorter and less subject to non-specific hybridization. Specificity of the disclosed method can also be increased by using a thermostable reverse transcriptase and performing reverse transcription at a high temperature. The disclosed method can also be used to detect reverse transcriptase activity in a sample and to identify inhibitors of reverse transcriptase. Also disclosed is a method for sequencing target RNA molecules using reverse transcriptase lacking an RNAse H function.

FIELD OF THE INVENTION

The present invention is in the general field of detection of nucleic acid sequences, and specifically in the field of detection of specific RNA sequences.

BACKGROUND OF THE INVENTION

The RNA or DNA for many genes, including those associated with disease states, and microorganisms and viruses have been isolated and sequenced. Nucleic acid probes based on such sequences are currently available to identify a large number of genes and infections. Nucleic acid probes are detectable nucleic acid sequences that hybridize to complementary RNA or DNA sequences in a test sample. Detection of the probe indicates the presence of a particular nucleic acid sequence in the test sample for which the probe is specific. In addition to aiding scientific research, DNA or RNA probes can be used to detect the presence of viruses and microorganisms such as bacteria, yeast and protozoa as well as genetic mutations linked to specific disorders in patient samples.

Grunstein, et al., Proc. Natl. Acad. Sci. USA 72:3961 (1975) and Southern, J. Mol. Biol. 98:503 (1975) describe hybridization techniques using radiolabelled nucleic acid probes. Nucleic acid hybridization probes have the advantages of high sensitivity and specificity over other detection methods and do riot require a viable organism. Hybridization probes are often labelled with a radioactive substance that can be easily detected.

Probes have been indirectly labelled in an attempt to avoid the problems associated with direct radioactive labelling. The most common method of indirect labelling is to attach biotin, a small vitamin, to the nucleic acid probe using a chemical or enzyme technique. Following hybridization to the specific RNA, the biotin is detected by reaction with streptavidin, a protein which binds biotin tightly and has been labelled with an enzyme or fluorochrome. Bound biotin-streptavidin complex can be detected by reaction with color-producing substrates and the fluorochrome can be seen when reacted with incident light of appropriate wavelength. However, indirect labelling of hybridization probes with biotin or other haptens often increases the "hydrophobicity" of the probe. The probe tends to interact non-specifically with materials other than the complementary nucleic acid target, leading to high background. High background reduces sensitivity and increases the likelihood of a false-positive result. Indirect labelling is also less sensitive than direct labelling because the labelling density is limited; only a small fraction of the bases are labelled giving a limiting number of sites for signal generation. An increase in the labelling density of a probe leads to increased non-specific binding, higher background, and ultimately, failure of the probe to hybridize with its target due to the interference of the hapten with base pairing. Indirectly labelled probes are therefore not well suited to clinical diagnosis.

Hybridization of a probe to the specific RNA sequences has been detected with the use of an intercalating agent such as acridine orange or ethidium bromide as described in U.S. Pat. No. 4,563,417 to Albarella et al. The intercalating agent becomes inserted between hybridized base pairs of probe and sample nucleic acids and causes the tertiary structure of the helix to unwind. An antibody specific for the newly formed antigenic determinant created by the intercalating agent and the unwound helix is detected by conventional means. This method lacks selectivity for the target hybrids because intercalating agents fail to recognize specific sequences. Furthermore, the antibodies recognize only the intercalating agent/nucleic acid complex, but do not detect a specific sequence. Therefore, additional selection or purification steps are required to prevent non-specific signal, making this approach poorly suited for clinical diagnosis.

Hybridization of the probe to the specific RNA sequences can also be detected with the aid of an antibody specific for a labelled probe as described in U.S. Pat. No. 4,743,535 to Carrico. The probe is labelled with a detectable substance such as flavin adenine dinucleotide (FAD) or a fluorescent agent. An antibody specific for the labelled probe, after it has hybridized to the specific RNA sequence, is detected by a biochemical reaction. This method of detection also creates non-specific binding and the likelihood of false-positive results and is not well suited for clinical screening.

Monoclonal antibodies to DNA-RNA hybrids are now available. U.S. Pat. No. 4,732,847 to Stuart et al. and the publication of Stuart et al., Proc. Natl. Acad. Sci. USA 78:3751 (1981) describe a method of hybridization detection of specific nucleic acid sequences on a solid surface involving a monoclonal antibody specific for a poly(A)-poly(dT) duplex. In Stuart, DNA/RNA hybrids are formed by annealing DNA or RNA sequences complementary to the sequence of interest.

A monoclonal antibody specific for DNA-RNA hybrids secreted by hybridoma HB 8730, is disclosed in U.S. Pat. No. 4,833,084 to Carrico et al. In Carrico, DNA/RNA hybrids are formed by annealing of RNA and DNA polynucleotides, or by transcription of DNA.

The isolation of anti-DNA-RNA hybridomas has improved the development of assays for genetic mutations linked to specific defects and the detection of bacterial and viral infections. However, assays utilizing these anti-hybrid monoclonal antibodies secreted from the hybridomas often employ large probes leading to a high level of non-specific binding causing false positive results. Boguslawski et al., J. Immunol. Methods 89:123-130 (1986) developed a hybridization assay using anti-hybrid coated polystyrene beads isolated on filter paper in an attempt to reduce non-specific binding and avoid complicated washing procedures.

Therefore, it would be useful to have a method to detect RNA sequences that is easy to use, highly specific, accurate and sensitive enough for clinical screenings.

Accordingly, it is an object of the present invention to provide a method of detecting specific RNA molecules in a sample with a relatively large signal based on specific hybridization of a relatively small primer.

It is further an object of the present invention to provide an assay having minimal false positives.

It is further an object of the invention to provide an assay to detect RNA indicative of an infection, a disease state, or predisposition to a disease.

It is further an object of the invention to provide a method for directly sequencing a target RNA molecule.

It is further an object of the invention to provide an assay for detecting reverse transcriptase activity and testing for reverse transcriptase inhibitors.

SUMMARY OF THE INVENTION

Disclosed is a method of detecting RNA molecules of interest in which reverse transcription primers unique to the RNA molecule of interest are used for reverse transcribing the RNA with a reverse transcriptase lacking RNAse H function and the resulting RNA/DNA hybrid is detected with an antibody specific for RNA/DNA hybrids. This method can be used to detect the presence of one or many specific RNA molecules which may be present in a sample, including RNA from different organisms (such as viruses, bacteria, fungi, plants, and animals), or RNA indicative of an infection, a disease state, or predisposition to a disease in an animal. The specificity of detection is increased relative to current detection methods involving probe hybridization since the reverse transcription primers are shorter and less subject to non-specific hybridization. Specificity of the disclosed method can also be increased by using a thermostable reverse transcriptase and performing reverse transcription at a high temperature.

The disclosed method can be used to detect the presence of one or many specific RNA molecules which may be present in a sample. The method can be used to detect, for example, continuous amplification reaction (CAR) generated RNA (Anthony et al., Cambridge Healthtech Institute's Second Annual Gene Quantification Meeting; Abstract #2 (1997)), RNA from different organisms (such as viruses, bacteria, fungi, plants, and animals), or RNA indicative of an infection, a disease state, or predisposition to a disease. For example, mRNA specific to tumor cells can be detected. The method is also useful for detecting a class of microorganisms or a group of related disease conditions.

The disclosed method allows easy detection of a relatively large signal based on specific hybridization of a relatively small primer. The specificity of detection is increased relative to detection methods involving probe hybridization because the reverse transcription primers employed in the disclosed method are shorter and are therefore less subject to non-specific hybridization. The specificity of the disclosed method can be increased by using a thermostable reverse transcriptase and performing reverse transcription at a high temperature, such as a temperature around the melting temperature of the hybrid between the primer and target RNA. The sensitivity of the disclosed method can also be increased by capturing the RNA/DNA hybrids and separating them from other nucleic acids in the sample. This can be mediated by a "capture tag" either on the reverse transcription primer or the RNA/DNA hybrid-specific antibody. The sensitivity of the disclosed method can also be increased by treating the sample with a single-strand-dependent ribonuclease, such as RNAse A, following reverse transcription to remove free RNA present in the sample.

Also disclosed is a method for sequencing target RNA molecules using reverse transcriptase lacking an RNAse H function. Target RNA molecules are isolated and are incubated with reverse transcriptase lacking an RNAse H function, appropriate oligodeoxyribonucleotide primers, and chain terminating dideoxynucleotides to form chain terminated RNA/DNA hybrids. The hybrids can then be isolated, detected, and analyzed in order to determine the sequence of the target RNA, preferably making use of an RNA/DNA hybrid-specific antibody. For example, the chain terminated RNA/DNA hybrids can be treated with RNase A to digest unhybridized RNA portions of the duplex, separated via electrophoresis, blotted on nitrocellulose membrane, and reacted with the anti-hybrid antibody in a fashion similar to a Western Blot.

The disclosed method also be used to detect reverse transcriptase activity in a sample. A preferred use of this form of the method is the screening and testing for reverse transcriptase inhibitors against organisms harboring this enzyme (such as HIV), even if the reverse transcriptase has an RNase H function. For example, a target RNA can be hybridized to appropriate 5'-biotin labeled oligodeoxyribonucleotide primers, reverse transcribed in the presence of chain terminating dideoxynucleotides and the presumptive enzyme inhibitor. The chain terminated RNA:DNA hybrids can then be treated with RNase A (to digest unhybridized RNA portions of the duplex), captured on a streptavidin coated solid phase, and detected with an RNA/DNA hybrid-specific antibody. Detection of hybrids is indicative of enzyme activity and lack of inhibition. The amount of hybrid present can also be used to estimate the level of inhibition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the sensitivity of an assay detecting RNA/DNA hybrids shown as the number of RNA molecules/reaction versus the signal-to-noise ratio minus 1.

DETAILED DESCRIPTION OF THE INVENTION

Disclosed is a method of detecting RNA sequences by reverse transcribing all or part of the target RNA molecule with a reverse transcriptase lacking an RNA/DNA hybrid-dependent exonuclease function (commonly referred to as an RNAse H function or component) and detecting the resulting RNA/DNA hybrid with an antibody specific for RNA/DNA hybrids. Also disclosed are a method for sequencing the target RNA and a method of determining reverse transcriptase activity.

Also disclosed is a method for sequencing target RNA molecules using reverse transcriptase lacking an RNAse H function. Target RNA molecules can be isolated and directly sequenced using RNase H- reverse transcriptase with appropriate oligodeoxyribonucleotide primers and chain terminating dideoxynucleotides (Mierendorf & Pfeffer, Methods. Enzymol., 15:563-566 (1987)). The chain terminated hybrids can be treated with RNase A to digest unhybridized RNA portions of the duplex, separated via electrophoresis, blotted on nitrocellulose membrane, and reacted with the anti-hybrid antibody in a fashion similar to a Western Blot. Direct sequencing of the transcripts allows for the analysis for the presence of mutations. Mutational analysis includes but is not limited to, point mutations, deletions and insertions. It will be understood by those skilled in the art that other forms of hybrid separation, immobilization, or solid phase can be utilized in the assay described above.

The disclosed method also allows easy detection of reverse transcriptase activity. This can be useful in the screening and testing for reverse transcriptase inhibitors against organisms harboring this enzyme (such as HIV), even if the reverse transcriptase has an RNAse H function. For example, a target RNA can be hybridized to appropriate 5'-biotin labeled oligodeoxyribonucleotide primers, reverse transcribed in the presence of chain terminating dideoxynucleotides and the presumptive enzyme inhibitor. The chain terminated RNA:DNA hybrids can then be treated with RNaseA (to digest unhybridized RNA portions of the duplex), captured on a streptavidin coated solid phase, and detected with an antibody specific for RNA/DNA hybrids. Detection of hybrids is indicative of enzyme activity and lack of inhibition. The amount of hybrid present can also be used to estimate the level of inhibition.

1. RNA Molecules of Interest

The disclosed method can be used to detect or sequence any RNA molecule or combination of RNA molecules in a sample. RNA molecules for detection, referred to herein as RNA molecules of interest, or target RNA molecules, are selected based on the needs and purpose of the detection. In general, an RNA molecule of interest can be chosen based on known criteria for selecting a nucleic acid sequence for detection. For example, a particular RNA molecule may be associated with a pathogen, a disease state, or a predisposition to a disease, and detection of such an RNA molecule can have a diagnostic value. For example, mRNA specific to tumor cells or normal cells can be detected.

The disclosed method can also be used to detect a plurality of different RNA molecules of interest in a sample from an individual organism. This is preferably accomplished by either screening for an RNA sequence that is present in each of the RNA molecules of interest, or by screening with multiple primers that are collectively complementary to regions on the RNA molecules of interest. The later approach is preferred for use in detecting, for example, some diseases or predispositions to disease that are associated with numerous different mutations to particular genes. In this case, it is preferred to screen for sequences that are complementary to the regions of the mutant RNA products of these genes that are characteristic of each of the mutations.

The disclosed method also allows the detection of the RNA of interest in different organisms, such as, fungus, bacteria, plants, animals and viruses by taking samples from each of the organisms of interest. The disclosed method also allows the detection of CAR generated RNA molecules.

Another RNA molecule of interest may be associated with a class of microorganisms or a group of related disease conditions. For this purpose, RNA molecules can be chosen based on their common presence in all or most of the members of such groups. For example, to detect mycobacteria, it is preferred to select an RNA molecule that is both unique to mycobacteria and present in most types of mycobacteria. The identification and selection of nucleic acid molecules for detection is a well developed area and such identification and selection criteria can be fully applied to the identification and selection of RNA molecules as targets of detection in the disclosed method.

RNA molecules of interest for use in the disclosed method can come from various sources, both natural or synthetic. Various types of RNA include messenger RNA, ribosomal RNA, nucleolar RNA, transfer RNA, viral RNA and heterogeneous nuclear RNA, or the like. In addition, whole naturally occurring entities or fragments thereof may be used.

2. Samples to Be Assayed for RNA

Samples to be used in the method of the invention are from any source identified as containing, or expected to contain, RNA. Preferred samples are those suspected or expected to contain one or more RNA molecules of interest. Samples can be, for example, subjects of a screen to determine which samples contain particular RNA molecules of interest, a body fluid or extract from a patient or other animal suspected of being infected or suffering from a disease condition, or an environmental sample (for example, soil or water) suspected of harboring a particular organism.

Alternatively, a sample for the disclosed method of the invention can be from any source containing or suspected of containing nucleic acid, where the nucleic acid has been treated to produce at least some RNA from the nucleic acid. The source of nucleic acid can be in purified or non-purified form. Preferred types of samples, or sources of samples, that are suitable for use in the disclosed method are those samples already known or identified as samples suitable for use in other methods of nucleic acid detection. Many such samples are known. For example, the sample may be from an agricultural or food product, or may be a human or veterinary clinical specimen. In a preferred embodiment, the sample is a biological fluid such as plasma, serum, blood, urine, sputum or the like. The sample may contain bacteria, yeast, viruses and the cells or tissues of higher organisms such as plants or animals, suspected of harboring an RNA of interest. Methods for the extraction and/or purification of RNA have been described, for example, by Maniatis et al., Molecular Cloning: A Laboratory Manual (New York, Cold Spring Harbor Laboratory, 1982).

Alternatively, samples suspected of containing RNA molecules of interest are coupled to a solid support or substrate. Samples provided in this manner are known as "solid-state samples." In this embodiment of the invention, target molecules or target sequences are delivered in a target sample or assay sample. A preferred form of solid-state sample is an array sample. An array sample is a solid-state sample to which multiple different target samples or assay samples have been coupled or adhered in an array, grid, or other organized pattern.

Substrates for use in solid-state samples include any solid material to which target molecules or target sequences can be coupled or adhered. This includes materials such as acrylamide, cellulose, nitrocellulose, polystyrene, polyethylene vinyl acetate, polypropylene, polymethacrylate, polyethylene, polyethylene oxide, glass, polysilicates, polycarbonates, teflon, fluorocarbons, nylon, silicon rubber, polyanhydrides, polyglycolic acid, polylactic acid, polyorthoesters, polypropylfumerate, collagen, glycosaminoglycans, and polyamino acids. Solid-state substrates can have any useful form including thin films or membranes, beads, bottles, dishes, slides, fibers, woven fibers, shaped polymers, particles, chips and microparticles. Preferred substrate forms for a solid-state sample are microtiter dishes and glass slides. The most preferred form of microtiter dish is the standard 96-well type.

Target RNA molecules immobilized on a substrate allow formation of RNA/DNA hybrids localized on the substrate. Such localization provides a convenient means of washing away reaction components that might interfere with subsequent detection steps, and a convenient way of assaying multiple different samples simultaneously. Diagnostic RNA/DNA hybrids can be independently formed at each site where a different sample is adhered. For immobilization of target sequences or other oligonucleotide molecules to form a solid-state sample, the methods described below can be used.

A preferred form of solid-state substrate is a glass slide to which up to 256 separate target or assay samples can be adhered as an array of small dots. Each dot is preferably from 0.1 to 2.5 mm in diameter, and most preferably around 2.5 mm in diameter. Such microarrays can be fabricated, for example, using the method described by Schena et al., Science 270:487-470 (1995). Briefly, microarrays can be fabricated on poly-L-lysine-coated microscope slides (Sigma) with an arraying machine fitted with one printing tip. The tip is loaded with 1 μl of a RNA sample (0.5 mg/ml) from, for example, 96-well microtiter plates and deposited approximately 0.005 μl per slide on multiple slides at the desired spacing. The printed slides can then be rehydrated for 2 hours in a humid chamber, snap-dried at 100° C. for 1 minute, rinsed in 0.1% SDS, and treated with 0.05% succinic anhydride prepared in buffer consisting of 50% 1-methyl-2-pyrrolidinone and 50% boric acid. The RNA on the slides can then be denatured in, for example, distilled water for 2 minutes at 90° C. immediately before use. Microarray solid-state samples can scanned with, for example, a laser fluorescent scanner with a computer-controlled XY stage and a microscope objective. A mixed gas, multiline laser allows sequential excitation of multiple fluorophores.

3. Primers

Once the RNA(s) of interest have been identified and the RNA sample(s) obtained, a primer is needed to begin the reverse transcription of the RNA molecule of interest as shown in FIG. 1. A primer of the disclosed method is an oligonucleotide having sequence complementary to a region on the RNA molecule of interest. As used herein, the complementary sequence of the primer is referred to as the "complementary portion". As used herein, the region on the RNA molecule of interest complementary to the primer is referred to as the "primer complement region". The primer complement region of an RNA molecule of interest can be any region of the RNA molecule of interest. It is preferred that the primer complement region of an RNA molecule be at some distance from the 5' end of the RNA molecule. This provides a longer region of RNA template between the site of primer hybridization and the end of the RNA molecule, thereby amplifying the amount of RNA/DNA hybrid to be detected.

In general, the primer complement region of an RNA molecule of interest is chosen based on known criteria for selecting an RNA sequence for detection. For example, to detect a particular RNA molecule from among other RNA molecules, it is preferred that the primer complement region is characteristic of, or unique to, the RNA molecule of interest. If it is desired that any of a class of RNA molecules be detected, it is preferred that the primer complement region is chosen to have a sequence that is the same or substantially the same in all of the RNA molecules of interest. Once a primer complement region is selected, the sequence of the primer is designed or chosen to be complementary to the chosen primer complement region of the molecule of interest. The nucleotide sequence of many RNA molecules are known. Any RNA molecule for which a sequence is known or for which a sequence can be derived can be detected using the disclosed method.

In the method of the invention the complementary portion of a primer has a length that supports specific and stable hybridization between the primer and the primer complement region. Generally a primer of the present invention comprises 10 to 100 nucleotides, but is preferably 15 to 30 nucleotides.

In one embodiment primers for use in the disclosed method are synthesized using established oligonucleotide synthesis methods. Methods to produce or synthesize oligonucleotides are well known in the art. Such methods range from standard enzymatic digestion followed by nucleotide fragment isolation (see for example, Sambrook et al., Molecular Cloning. A Laboratory Manual, 2nd Edition (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989) Chapters 5, 6) to purely synthetic methods, for example, by the cyanoethyl phosphoramidite method using a Milligen or Beckman System 1Plus DNA synthesizer (for example, Model 8700 automated synthesizer of Milligen-Biosearch, Burlington, Mass. or ABI Model 380B). Synthetic methods useful for making oligonucleotides are also described by Ikuta et al., Ann. Rev. Biochem. 53:323-356 (1984), (phosphotriester and phosphite-triester methods), and Narang et al., Methods Enzymol., 65:610-620 (1980), (phosphotriester method).

Hybridization of the primer to the RNA molecule of interest can be carried out under any suitable conditions, and preferably under conditions which favor hybrid DNA complexing between RNA and DNA. See for example, Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd Edition (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989).

4. Reverse Transcription

In the disclosed method, reverse transcription is carried out using a reverse transcriptase, preferably a reverse transcriptase lacking RNAse H function. The reaction mixture including the RNA molecule of interest, the hybridized primer, and the reverse transcriptase is then incubated under conditions to allow reverse transcription of the RNA molecule of interest and formation of DNA/RNA hybrids. Examples of reverse transcriptases that can be used in the disclosed method, or that can be adapted for use in the disclosed method are listed in Table 1. Preferred reverse transcriptases for use in the present method include reverse transcriptases 18053-017, 18064-014 and 18064-071 from Life Technology; reverse transcriptases M5301 and M5302 from Promega; and reverse transcriptase 600085 from Strategene; each disclosed in Table 1.

                                      TABLE 1                                      __________________________________________________________________________     SPECIFIC                                               SUPPLIER                                                                        ACTIVITY UNITS                                                                DEFINITION                                                                     PREPARATION FORM                                                               ADDITIONAL                                                                     ACTIVITIES CATALOG                                                             NO.                     __________________________________________________________________________            1 unit incorporates 1 nmol TTP                                                                0.2M KPO.sub.4, 2 mM DTT, 0.2% Triton                                                            No detectable RNase,                                                                          Adv Biotech                                                                      into acid-insolub                                                            le form/10 min at                                                              X-100, 50%                                                                     glycerol, pH 7.2                                                               exonuclease                                                                    AB-0321                    35° C. using poly(A).oligo dT.sub.12-28   AB-0321b                      as substrate                                                                  26,700 U/mL 1 unit incorporates 1 nmol dTMP 200 mM KPO.sub.4, 2 mM DTT,                                                             0.2% Triton No                                                                 detectable                                                                     endonuclease, ACS                                                              Heidelb                    into acid-precipitable form/10 min X-100, 50% glycerol, pH 7.2 RNase                                                               F00750S                    at pH 8.3, 37° C.   F00750M                                            10-20 U/μl 1 unit incorporates 1.0 nmol [.sup.3 H]- 0.2M KPO.sub.4,                                                              2.0 mM DTT, 0.2%                                                               Triton No detectabl                                                            e endonuclease,                                                                Amersham                   JTTP into acid-insoluble X-100, 50% glycerol, pH 7.2 exonuclease,                                                                  RNase E 70041Y                                                                   products/10 min                                                              at 37° C.                                                               E 70041Z                  >50,000 U/mg; 1 unit incorporates 1 nmol [.sup.3 H]- 200 mM KPO.sub.4,                                                              2 mM DTT, 0.2%                                                                 Triton No detectabl                                                            e nonspecific                                                                  Boehringer                                                                      >20,000 U/mL dTMP                                                             into acid-precipita                                                            ble X-100, 50%                                                                 glycerol, pH 7.2                                                               RNases, nonspecific                                                             DNases (gel                                                                   10911B                     products/10 min at 37° C. using  electrophoresis) 1495062                                                                     poly(A).d[pT].sub                                                            .15 as template                                                                primer                    30,000 u/mL 1 unit incorporates 1 nmol [.sup.3 H]- Solution containing                                                              0.2M KPO.sub.4,                                                                2.0 mM No detectabl                                                            e nonspecific ICN                                                                TMP into nucleic                                                             acid product/10                                                                DTT, 0.2% Triton                                                               X-100, 50%                                                                     glycerol, nuclease                                                             855928                     min at 37° C. pH 7.2  855929                                           13 U/μL 1 unit incorporates 1 nmol dTNP 0.2M KPO.sub.4, 2 mM DTT,                                                                0.2% Triton No                                                                 detectable RNase,                                                              exogenous, NBL                                                                 Gene                       into a TCA-insoluble product/10 X-100, 50% glycerol, pH 7.2 nicking or                                                             degradation of RNA                                                             020704                     min at pH 8.3, 37° C.                                                  30 U/μL 1 unit incorporates 1 nmol dTNP 0.2M KPO.sub.4, 2 mM DTT,                                                                0.2% Triton No                                                                 detectable RNase,                                                              NBL Gene                   into a TCA-insoluble product/10 X-100, 50% glycerol, pH 7.2; for high                                                              exonuclease,                                                                   endonuclease,                                                                  020703                     min at pH 8.3, 37° C. efficiency synthesis of full length cDNA                                                              nicking                     in the 6-10 kilobase range                                                   >20,000 U/mg 1 unit incorporates 1 nmol dNTP 200 mM KPO.sub.4, 2 mM                                                                 DTT, 0.2% Triton                                                               No detectable                                                                  RNase, DNase Oncor        10,000-20,000 into DE-81 adsorbable form/10 X-100, 50% glycerol, pH 7.2                                                              120111                   U/mL min at 37° C.   120112                                             25,000-50,000 1 unit incorporates 1 nmol dTMP Molecular biology grade;                                                              homogeneous No                                                                 detectable RNase,                                                              DNase, Pharmacia                                                                U/mg protein;                                                                 into acid-insoluble                                                             product/10 min                                                                purity; solution                                                               containing 0.2M                                                                KPO.sub.4, nickase                                                             27-0922-01                                                                      10,000-20,000 at                                                              pH 8.3, 37°                                                             C. using 2.0 mM                                                                DTT, 0.2% Triton                                                               X-100, 50%                                                                     27-0922-02                                                                      U/mL poly(A)p[dT].                                                            sub.12-18 as                                                                   template glycerol,                                                             pH 7.2                     primer                                                                        10,000-70,000 1 unit incorporates 1.0 nmol [.sup.3 H]- Purified; 20 mM                                                              KPO.sub.4, 2 mM                                                                DTT, No detectable                                                             endonuclease,                                                                  Stratagene                                                                      U/mL dTTP into                                                                acid-insoluble                                                                 0.2% Triton X-100,                                                             50% glycerol, pH                                                               nonspecific RNase                                                              600081                     product/10 min at 37° C. 7.2  600082                                   >40,000 U/mg; 1 unit incorporates 1 nmol TMP in Cooiassie Blue shows a                                                              single band No                                                                 detectable                                                                     nonspecific                                                                    Boehringer                                                                      >20,000 U/mL                                                                  acid-insoluble                                                                 product/10 min at                                                              purity; 50 mM                                                                  Tris.HCl, 10 mM                                                                DTT, RNases,                                                                   nonspecific DNases                                                             (gel 1062603                                                                     37° C.                                                                with poly(A.[dT].su                                                            b.15 as 100 mM                                                                 NaCl, 0.05%                                                                    polydocanol, 1                                                                 electrophoresis)                                                                 substrate mM                                                                 EDTA, 50% glycerol,                                                             pH 8.4                   50-250 U/μL 1 unit incorporates 1 nmol >90% purity by SDS-PAGE; 20                                                               mM No detectable                                                               non-specific ss-                                                               Amersham                   deoxynucleotides into acid- Tris.HCl, 0.1M NaCl, 0.1 mM EDTA, and                                                                  ds-endonuclease, E                                                             70456Y                     precipitable material/10 min at 1 mM DTT, 0.01% NP40, 50% glycerol,                                                                exonuclease, RNase                                                             E 70456Z                   37° C. using pH 7.5                                                     poly(rA).oligo[dT].sub.12-18                                                   as template primer                                                            50,000 U/mL 1 unit incorporates 1 nmol TTP 50 mM Tris.HCl, 0.1 mM DTT,                                                              100 mM No detectabl                                                            e endonuclease,                                                                ACS Heldelb                                                                      into acid-insolub                                                            le form/10 min at                                                              NaCl, 1 mM EDTA,                                                               0.1% NP40, 50%                                                                 RNase F00755S                                                                    pH 8.0, 37.degree                                                            . C. using                                                                     glycerol, pH 8.3                                                               F00755M                    poly(rA).oligo[dT] as template                                                 primer                                                                        200 U/μL 1 unit incorporates 1 nmol Purity by SDS-PAGE, 250 mM No                                                                detectable RNase H                                                             Life Technol                                                                     deoxyribonucleoti                                                            de into acid-                                                                  Tris.HCl, 15 mM                                                                MgCl.sub.2, 375 mM                                                             KCl,  18053-017                                                                  precipitable                                                                 material/10 min at                                                             pH pH 8.3 and 100                                                              mM DTT                     8.3, 37° C. using(poly(A)-                                              oligo.[dT].sub.12-18 as template primer                                       200 U/μL 1 unit incorporates 1 nmol Purity by SDS-PAGE, 250 mM Tris-                                                             No detectable                                                                  RNase H Life                                                                   Technol                    deoxyribonucleotide into acid- HCl, 15 mM MgCl.sub.2, 375 mM KCl, pH                                                               18064-014                  precipitable material/10 min at pH 8.3 and 100 mM DTT  18064-071                                                                     8.3, 37°                                                              C. using poly(A)-                                                                oligo.[dT].sub.12                                                            -18 as template                                                                primer                    200 U/μL 1 unit incorporates 1 nmol   Life Technol                           deoxyribonucleotide into acid -   28025-013                                    precipitable material/10 min at pH   28025-021                                 8.3, 37° C. using poly(A)-                                              oligo.[dT].sub.12-18 as template primer                                       25,000 U/mL 1 unit incorporates 10 nmol TTP 0.1 mM NaCl, 50 mM Tris.Hcl,                                                             5 mM No detectable                                                             endonuclease, NE                                                              Biolabs                    into acid-insoluble material/10 min DTT, 1 mM EDTA, 0.1% NP40, 50%                                                                 RNase 253S                                                                       at 37° C.                                                             using poly(rA).olig                                                            o(dT) glycerol, pH                                                             7.6  253L                  as template primer                                                            ≧5000 U/mg 1 unit incorporates 1.0 nmol [.sup.3 H]- Recombinant;                                                             99% by HPLC, SDS-                                                              No detectable                                                                  nuclease Boehringer       protein TMP into acid-insoluble PAGE; lyophilized containing 0.2% BSA                                                               1465333                    products/10 min at 37° C. using as stabilizer                           poly(A).d[pT].sup.15 as substrate                                             20-40 U/μL 1 unit incorporates 1 nmol Overproducer; 50 mM Tris.HCl,                                                              0.1M No detectable                                                             endo and Fermentas         deoxyribonucleotide into DE-81 NaCl, 0.1% Triton X-100, 1 mM EDTA,                                                                 exodeoxyribonucleas                                                            es, RNases EP0351                                                                absorbable                                                                   form/10 min at                                                                 37° C. 5 mM                                                             DTT, 50% glycerol,                                                             pH 8.3  EP0352                                                                  100-200 U/μL 1                                                             unit incorporates                                                              1 nmol dTTP                                                                    Recombinant;                                                                   ≧90% purity                                                             by SDS gel; No                                                                 detectable RNase H                                                             Promega                    into acid-insoluble form/10 min at 5X reaction buffer; 250 mM Tris.HCl,                                                             <1% DNase M5301                                                                 pH 8.3, 37.degree                                                            . C. 375 mM KCl,                                                               15 mM MgCl.sub.3,                                                              50 mM <3% RNase                                                                M5302                       DTT, pH 8.3 ≧90% supercoiled plasmid                                   1 unit incorporates 1 nmol TTP 50 mM Tris.HCl, 0.1M NaCl, 1 mM No                                                                  detectable RNase,                                                              Adv Biotech                                                                      into acid-insolub                                                            le form/10 min at                                                              EDTA, 5 nM DTT,                                                                0.1% Triton X-100,                                                             exonuclease                                                                    AB-0322                    37° C. using poly(A).oligo[dt].sub.11-18 50% glycerol, pH 8.3                                                               AB-0322b                   as substrate                                                                   1 unit incorporates 1 nmol dTTP 50 mM Tris.HCl, 5.0 mM DTT, 1.0 mM No                                                              detectable                                                                     endocnulease,                                                                  CHIMERx                    into acid-insoluble form/10 min at EDTA, 0.1M NaCl, 0.1% NP40, 50%                                                                 RNase 1375-01                                                                    37° C.                                                                glycerol, pH 8.0                                                               1375-02                   35,000 U/mg 1 unit incorporates 10 nmol dTTP Solution containing 50%                                                                glycerol, 50 mM No                                                             detectable RNase,                                                              Epicentre                  into acid-insoluble material/10 min Tris.HCl, 0.1M NaCl, 0.1 mM EDTA,                                                              endonuclease,                                                                  exonucleolytic                                                                 M4425H                     at pH 8.6, 37° C. using 1 mM DTT, 0.1% Triton X-100, pH 7.5                                                                 DNase, protease                                                                M4410H                     oligo.(dT).sub.12-18 primed poly(A).sub.n as                                   template                                                                       1 unit incorporates 1 nmol labeled Solution containing 0.1 mM NaCl, 50                                                              ICN                       dATP into acid-insoluble mM Tris.HCl, 1 mM EDTA, 5 mM  152020                  material/10 min at 37° C. DTT, 0.1% NP40, 50% glycerol, pH 8.0                                                                1 unit incorporat                                                            es 1 nmol TMP 50                                                               mM Tris.HCl, 0.1M                                                              NaCl, 5 mM No                                                                  detectable RNase,                                                              DNase Oncor                                                                      into DE-81                                                                   adsorbable form/10                                                             DTT, 1 mM EDTA,                                                                0.1% Triton X-100,                                                              120301                    min at 37° C. using polyA- 50% glycerol, pH 8.3  120302                                                                       oligodT.sub.12-18                                                             as substrate                                                                   50,000-95,000 1                                                               unit incorporates                                                              1 nmol dTMP                                                                    Molecular biology                                                              grade; homogeneous                                                             No detectable                                                                  RNase, DNase,                                                                  Pharmacia                 U/mg protein; into acid-insoluble product/10 min purity; solution                                                                   containing 50 mM                                                               nickase 27-0925-01        10,000-20,000 at pH 8.3, 37° C. using Tris.HCl, 0.1M NaCl, 1 mM                                                              EDTA, 5  27-0925-02       U/mL poly(rA).p[dT].sub.12-18 as template mM DTT, 0.1% Triton X-100,                                                                50%                        primer glycerol, pH 8.3                                                       50,000 U/mL 1 unit incorporates 1.0 nmol 50 mM Tris.HCl, 5 mM DTT, 1 mM                                                             No detectable                                                                  RNase H, DNase,                                                                Strategene                                                                       [.sup.3 H]TTP                                                                into acid-insoluble                                                             EDTA, 100 nM                                                                  NaCl, 0.1% NP40,                                                               nonspecific RNase                                                              600085                     product/10 min at 37° C. 50% glycerol, pH 8.0                          10-30 U/μL 1 unit incorporates 1 nmol 200 mM KPO.sub.4, 2 mM DTT,                                                                0.2% NP40, No                                                                  detectable                                                                     non-specific                                                                   Amersham                   [.sup.3 H]dTMP/10 min at 37° C. with 50% glycerol, pH 7.2                                                                   nuclease E 2610Y                                                                 poly(rA).oligo(dT                                                            ) as template   E                                                              2610Z                      primer                                                                        400 U and 1 unit incorporates 1 nmol Solution containing 200 mM                                                                     KPO.sub.4, 2 mM No                                                             detectable                                                                     nuclease TaKaRa                                                                 1600 U (.sup.3                                                                H]dTMP/10 min at                                                               pH 8.3, DTT, 0.2%                                                              NP40, 50% glycerol,                                                             pH 7.2  2610                                                                    37° C.                                                                with poly(rA).oligo                                                            (dT) as                    template primer                                                                Transcriptase: 1 unit incorporates Solution containing 50% glycerol,                                                               50 mM  Epicentre                                                                 4 nmol dTTP into                                                             acid insoluble                                                                 Tris.HCl, 0.1M                                                                 NaCl, 0.1 mM EDTA,                                                              Retrotherm ™                                                                 material/30 nun                                                              at pH 8.3,                                                                     45° C. 1 mM                                                             DTT, 0.5% Tween                                                                20, 0.5%  RT                                                                     using oligo(dT)18                                                            -primed poly(A).sub                                                            .n NP40, pH 7.5;                                                               no detectable DNA                                                              exo-  R19250                                                                     as template; DNA                                                             Polymerase: 1 and                                                              endonuclease,                                                                  protease, RNase                                                                R19500                     unit incorporates 10 nmol dNTP   R1910H                                        into acid insoluble material/30 min                                            at pH 8.3, 74° C.                                                       1 unit incorporates 1 nmol dTTP 50 mM Tris.HCl, 5.0 mM DTT, 0.1 mM No                                                              detectable                                                                     endonuclease, 3'-                                                              CHIMERx                    into acid-insoluble form/10 min at EDTA, 50% glycerol, stabilizers, pH                                                             7.5 exonuclease,                                                               5'-exonuclease/5'-                                                             1374-01                    50° C.  phosphatase, nonspecific RNase, 1374-02                           ss- and ds-DNase                                                          __________________________________________________________________________

Reverse transcription can generally be performed at any temperature within the functional temperature range of the reverse transcriptase. Preferably, the temperature of incubation is any temperature at which the reverse transcriptase is functional and the primer remains hybridized to the RNA molecule. For non-thermophilic reverse transcriptases, preferred temperatures are those temperatures that are at or around the optimum temperature for the reverse transcriptase. For most non-thermophilic reverse transcriptases this temperature will be between about 25° C. and 45° C.

In a preferred embodiment, a thermophilic reverse transcriptase is used for increasing selectivity. The highest temperature at which a thermophilic reverse transcriptase is functional can be quite high. For this reason, preferred temperature ranges for reverse transcription when a thermophilic reverse transcriptase is used are most conveniently described in terms of the calculated melting temperature of a hybrid between the RNA molecule of interest and the primer. Such a melting temperature is referred to herein as the RNA/primer melting temperature (R/P Tm). Preferred ranges include a temperature from 20° C. below the melting temperature of a hybrid between the RNA molecule of interest and the primer and 5° C. above the melting temperature of a hybrid between the RNA molecule of interest and the primer. Other preferred ranges when using a thermophilic reverse transcriptase include from 20° C. below R/P Tm to R/P Tm, from 15° C. below R/P Tm to R/P Tm, from 10° C. below R/P Tm to R/P Tm, from 7° C. below R/P Tm to R/P Tm, from 5° C. below R/P Tm to R/P Tm, from 3° C. below R/P Tm to R/P Tm, from 20° C. below R/P Tm to 5° C. below R/P Tm, from 15° C. below R/P Tm to 5° C. below R/P Tm, from 10° C. below R/P Tm to 5° C. below R/P Tm, from 7° C. below R/P Tm to 5° C. below R/P Tm, from 7° C. below R/P Tm to 3° C. below R/P Tm, and from 5° C. below R/P Tm to 3° C. below R/P Tm. It is specifically noted that every specific, but unnamed, range within the enumerated ranges above is contemplated as an alternative preferred range. Preferred temperatures for reverse transcription include about 20° C. below R/P Tm, about 15° C. below R/P Tm, about 12° C. below R/P Tm, about 10° C. below R/P Tm, about 7° C. below R/P Tm, about 5° C. below R/P Tm, about 3° C. below R/P Tm, 20° C. below R/P Tm, 15° C. below R/P Tm, 12° C. below R/P Tm, 10° C. below R/P Tm, 7° C. below R/P Tm, 5° C. below R/P Tm, and 3° C. below R/P Tm. In general, the closer the temperature is to the R/P Tm, the greater the degree of discrimination there will be between specific and non-specific hybrids of the RNA and primer. If the temperature is close to the R/P Tm, however, decreased stability of specific hybrids may cause priming to be less efficient.

R/P Tm can be determined either by calculation or by empirical measurement. For calculating R/P Tm, any established formula for calculating stability of nucleic acid hybrids can be used. A preferred formula for calculating R/P Tm is Tm=81.5+16.6(log M)+0.41(% G+C)-0.72(% formamide), which was derived from studies on the stability of perfectly-matched DNA:DNA hybrids. For RNA:DNA hybrids, incorporating formamide concentration in the formula does not hold because the relationship between formamide concentration and the depression of Tm is not linear. At 80% formamide, RNA:DNA hybrids are more stable than DNA:DNA hybrids, increasing the Tm by about 10 to 30° C. depending on the sequence (Hames & Higgins, Nucleic Acid Hybridisation: A Practical Approach (IRL Press Limited, Oxford, England. 1985)). Carrying out the reaction in 80% formamide can therefore also be used to suppress formation of DNA:DNA duplexes, to preferentially select RNA:DNA hybrids, and to estimate the Tm for R/P. Because the empirically derived formulas for the estimation of RNA:DNA hybrid Tm may not be as accurate for short DNA primers, the hybridization temperature is preferably determined by assessing hybrid stability in 0.1-0.4 M monovalent cation at temperatures ranging from 40 to 60° C. R/P Tm can also be determined empirically (Lesnick and Freier, Biochemistry 34:10807-10815 (1995), McGraw et al., Biotechniques 8:674-678 (1990), and Rychlik et al., Nucleic Acids Res. 18:6409-6412 (1990)).

As used herein, a thermophilic reverse transcriptase is any reverse transcriptase that retains at least 5% of its maximum activity at any temperature above 50° C. or which has an optimal temperature of at least 50° C. Preferred reverse transcriptases are those which have an optimal temperature of at least 50° C. As used herein, maximum activity of a reverse transcriptase is defined as the activity, as measured in the assay described below, that a given reverse transcriptase exhibits at its optimal temperature. As used herein, optimal temperature of a reverse transcriptase is defined as the temperature at which the activity of the reverse transcriptase is greatest, as measured in the assay described below. The optimal temperature for a given reverse transcriptase can be determined by measuring its activity in the following assay at various temperatures. In general, an optimal temperature need be determined only to within a range so that assays need only be performed at intervals of 5 to 10 degrees.

5. Antibodies Specific for RNA/DNA Hybrids

As used herein, the term "antibody" is intended to include whole, intact antibodies, antibody fragments, polyfunctional antibody aggregates, or in general any antibody-derived substance that comprises at least one antibody combining site having the characteristics described herein. Antibodies of any of the known classes and subclasses of immunoglobulins are contemplated for example, IgG, IgM, and so forth, as well as active fragments such as the IgG fragments conventionally known as Fab, F(ab'), and F(ab')₂.

A variety of particular techniques can be used to obtain suitable antibodies specific for RNA/DNA hybrids. See for example, the methods described in U.S Pat. No. 4,833,084 to Carrico, U.S. Pat. No. 4,732,847 to Stuart et al. and the publication of Stuart et al., Proc. Natl. Acad. Sci. USA 78:3751 (1981), each of which is herein incorporated by reference.

Any anti-hybrid antibodies specific for a double-stranded RNA/DNA hybrids may be used to directly detect the hybrid of the invention. In a preferred embodiment of the present assay, a polyclonal anti-RNA/DNA hybrid antibody is derived from goats immunized with an RNA/DNA hybrid. Hybrid-specific antibody is purified from the goat serum by affinity purification against RNA/DNA hybrid immobilized on a solid support. Monoclonal antibody prepared using standard techniques can be used in place of the polyclonal antibodies.

The preferred antibody for RNA/DNA hybrids is prepared by the method of Kitawaga, Y. and Stollar, B.D., Mol. Immunology 19:413-420 (1982) or according to the method set forth in U.S. Pat. No. 4,732,847, issued Mar. 22, 1988 to Stuart et al., both of which are incorporated herein by reference.

Hybridized RNA-DNA samples are incubated with the antibodies for a sufficient amount of time to allow conjugation of the hybrids. The hybrids are bound to the antibodies by incubation for 5 minutes to 24 hours at 15 to 65° C. on a platform shaker with a shaking speed of 0 to 1500 rpm. Preferably, the incubation time is 30 to 120 minutes at 20 to 40° C., with shaking at 300 to 1200 rpm. Most preferably, binding occurs with incubation at one hour at room temperature with vigorous shaking on a rotary platform shaker with a rotary shaking speed between approximately 300 and 1000 rpm. It will be understood by those skilled in the art that the incubation time, temperature, and shaking can be varied to achieve alternative capture kinetics as desired.

6. Detection of Antibody-Hybrid Conjugate

The identification of the presence of the hybrids may now be achieved by employing monoclonal antibodies specific for the hybrid complex. Detection can be achieved by labeling either the monoclonal antibody specific for the hybrid DNA-RNA complex, or by employing labeled antibodies which bind to the anticomplex. For example, where the monoclonal antibody is derived from a mouse, antibodies to mouse antibodies, for example rabbit anti(-mouse IgG), could be labeled so as to bind to any anticomplex bound to the complex bound to the solid support.

A wide variety of labels have been used in other environments which would be applicable here. One of the more common labels is radionuclides, which can be used with autoradiography to visualize the areas of binding. Another label is a fluorescer such as fluorescein, mercocyanine, or rhodamine, which by irradiation with light of excitation, the presence of fluorescence can be monitored. Alternatively, an enzyme can be used which results in a product which can be detected and localized in the area of the enzyme. A large number of dyes or metals capable of reduction can be employed to provide detection. Common enzymes include horseradish peroxidase, glucose oxidase, β-galactosidase, or the like. The particular label or manner in which the detectable signal is observed is not critical to this invention. By employing antibodies to the anticomplex, the number of labels associated with a particular binding of the anticomplex to the complex can be greatly amplified.

In one embodiment the antibody conjugate is produced by well known means such as direct reduction of the monoclonal antibody with dithiothreitol, (DTT, Sigma Chemical Company, St. Louis, Mo.) to yield monovalent antibody fragments. The reduced antibody is then directly conjugated to maleimated alkaline phosphatase by the methods of Ishikawa et al., J. Immunoassay 4:209-237 (1983) and Means, G. and Feeney, R., Bioconj. Chem. 1: 2-12 (1990) and the resulting conjugate is purified by HPLC.

Alternatively, antibody-hybrid conjugate can be detected indirectly using an unlabelled anti-hybrid antibody for which a labelled antibody is specific. For example, the anti-hybrid antibody can be a mouse immunoglobulin that is detected by a labelled goat anti-mouse antibody. In addition, antibody-hybrid conjugates can be detected by conjugating the primer used for reverse transcription to a label, such as an enzyme, or to a hapten, such as biotin that is then detected with a labelled anti-hapten antibody. As described above, conjugated hybrid can also be detected with a direct labelled RNA probe, such as an enzyme-conjugated hybridization probe, or a hapten-modified probe that is subsequently detected by a labelled anti-hapten antibody.

Bound conjugate is subsequently detected by colorimetry or chemiluminescence as described by Coutlee, et al., J. Clin. Microbiol. 27:1002-1007 (1989). Preferably, bound alkaline phosphatase conjugate is detected by chemiluminescence with a reagent such as a Lumi-Phos™ 530 reagent (Lumigen, Detroit, Mich.) using a detector such as an E/Lumina™ luminometer (Source Scientific Systems, Inc., Garden Grove, Calif.) or an Optocomp I™ Luminometer (MGM Instruments, Hamden, Conn.).

To facilitate detection of resulting binding of the antibody to the hybrid, the antibody will normally either be labeled with a detectable chemical group and/or be in an immobilized or immobilizable form. Examples of detectable chemical groups that can serve as labels are enzymatically active groups, such as coenzymes, enzyme substrates, enzyme inhibitors, and enzymes themselves, fluorescers, chromophores, luminescers, specifically bindable ligands such as biotin or haptens which are detectable by binding of labeled avidin or labeled anti-hapten antibodies, and radioisotopes. When the antibody is labeled, other means can be used to immobilize the hybrid to that detection of label associated with the ultimate solid-phase or liquid-phase can be related to the presence of the sequence of interest in the sample. The reverse can also be followed, use of the antibody to immobilize hybrids with other means being used to label them. Also, antibody to the hybrid can be both labeled and immobilized or immobilizable such that the antibody serves both functions of labeling and immobilization.

7. Capture to Increase Sensitivity

In another embodiment, primers for use in the disclosed method also include a "capture tag." In a preferred embodiment "capture tags" separate RNA/DNA hybrids produced in the disclosed method from other components in an assay, including, for example, other nucleic acid molecules and buffer components. The use of capture tags increases the sensitivity of the method of the invention. As used herein, a capture tag is any compound that can be associated with an RNA/DNA hybrid and which can be used to separate compounds or complexes having the capture tag from those that do not. A capture tag may be on the reverse transcription primer or the RNA/DNA hybrid-specific antibody. Preferably, a capture tag is a compound, such as a ligand or hapten, that binds to or interacts with another compound, such as ligand-binding molecule or an antibody. It is also preferred that such interaction between the capture tag and the "capturing component" be a specific interaction, such as between a hapten and an antibody or a ligand and a ligand-binding molecule.

Suitable capture tags include hapten or ligand molecules that can be coupled to, for example, reverse transcription primers or antibodies used to detect RNA/DNA hybrids. Preferred capture tags, described in the context of nucleic acid probes, have been described by Syvnen et al., Nucleic Acids Res., 14:5037 (1986). Preferred capture tags include biotin, which can be incorporated into nucleic acids. One or more of the components of the conjugate can be "captured", adhered to or coupled to a substrate, that is, a solid support for the conjugate. It is preferred that either the primer or antibody be coupled to a substrate. In particular embodiments, such support allows simplified washing and handling of the components, and allows automation of all or part of the assay.

Adhering or coupling conjugate components to a substrate may be accomplished in several ways. In one embodiment, "capture docks" are adhered or coupled to the substrate.

The capture docks mediate adherence of a conjugate component such as a primer or antibody by binding to, or interacting with, a "capture tag" on the component. Capture docks immobilized on a substrate allow capture of the conjugate component on the substrate. Such capture provides a convenient means of washing away reaction components that might interfere with subsequent detection steps. By attaching different capture docks to different regions of a substrate different capture tags attached to different conjugate components, can be captured at different, and therefore diagnostic, locations on the substrate. For example, in a microtiter plate multiplex assay, capture docks specific for up to 96 different capture tags can be immobilized on a microtiter plate, each in a different well. Capture and detection will occur only in those wells corresponding to capture tags for which the corresponding RNA molecules were present in a sample.

Substrates for use in the disclosed method can include any solid material to which components of the assay can be adhered or coupled. Examples of substrates include, but are not limited to, materials such as acrylamide, cellulose, nitrocellulose, glass, polystyrene, polyethylene vinyl acetate, polypropylene, polymethacrylate, polyethylene, polyethylene oxide, polysilicates, polycarbonates, teflon, fluorocarbons, nylon, silicon rubber, polyanhydrides, polyglycolic acid, polylactic acid, polyorthoesters, polypropylfumerate, collagen, glycosaminoglycans, and polyamino acids. Substrates can have any useful form including thin films or membranes, beads, bottles, dishes, fibers, woven fibers, shaped polymers, particles and microparticles. Preferred forms of substrates are plates and beads. The most preferred form of beads are magnetic beads.

In one embodiment, the capture dock is an oligonucleotide. Methods for immobilizing and coupling oligonucleotides to substrates are well established. For example, attachment methods suitable for the present invention are described by Pease et al., Proc. Natl. Acad. Sci. USA 91(11):5022-5026 (1994), and Khrapko et al., Mol Biol (Mosk) (USSR) 25:718-730 (1991). A method for immobilization of 3'-amine oligonucleotides on casein-coated slides, also suitable for the present invention, is described by Stimpson et al., Proc. Natl. Acad. Sci. USA 92:6379-6383 (1995). A preferred method of attaching oligonucleotides to solid-state substrates is described by Guo et al., Nucleic Acids Res. 22:5456-5465 (1994).

In another embodiment, the capture dock is the anti-hybrid antibody. Such antibodies can be specific for a RNA/DNA hybrid molecule of interest. Captured hybrid molecules of interest can then be detected by binding of a second, reporter antibody, followed by binding proteins, such as RCA (ricinus communis agglutinin) or RCT (ricinus communis toxin). Such a use of antibodies with a substrate allows RCA assays to be developed for the detection of any molecule for which antibodies can be generated.

Methods for immobilizing antibodies to substrates are well established. Immobilization can be accomplished by attachment, for example, to aminated surfaces, carboxylated surfaces or hydroxylated surfaces using standard immobilization chemistries. Examples of attachment agents are cyanoglen bromide, succinimide, aldehydes, tosyl chloride, avidin-biotin, photocrosslinkable agents, epoxides and maleimides. A preferred attachment agent is glutaraldehyde. These and other attachment agents, as well as methods for their use in attachment, are described in Protein immobilization: fundamentals and applications, Richard F. Taylor, ed. (M. Dekker, New York, 1991), Johnstone and Thorpe, Immunochemistry In Practice (Blackwell Scientific Publications, Oxford, England, 1987) pages 209-216 and 241-242, and Immobilized Affinity Ligands, Craig T. Hermanson et al., eds. (Academic Press, New York, 1992). Antibodies can be attached to a substrate by chemically cross-linking a free amino group on the antibody to reactive side groups present within the substrate. For example, antibodies may be chemically cross-linked to a substrate that contains free amino or carboxyl groups using glutaraldehyde or carbodiimides as cross-linker agents. In this method, aqueous solutions containing free antibodies are incubated with the solid-state substrate in the presence of glutaraldehyde or carbodiimide. For crosslinking with glutaraldehyde the reactants can be incubated with 2% glutaraldehyde by volume in a buffered solution such as 0.1 M sodium cacodylate at pH 7.4. Other standard immobilization chemistries are known by those of skill in the art.

Copending application serial number 08/183,154, filed Sep. 30, 1997, describing direct and indirect immobilization of anti-hybrid antibodies, is hereby incorporated by reference.

8. RNAse A to Increase Sensitivity

In another embodiment, the sensitivity of the disclosed method is increased by treating the hybrid sample (following reverse transcription) with a single-strand-dependent ribonuclease, such as RNAse A, to remove free RNA present in the sample.

A solution preferably containing RNAse at a concentration between 0.01 and 1 mg/ml and the above described conjugated anti-hybrid molecule is incubated for approximately 5 minutes to 24 hours at temperature between 4 and 45° C. The purpose of the RNA digestion enzyme is to degrade non-specific free RNA that may be bound to the tube. It is important to remove the non-specific free RNA because secondary structures in the nucleic acid can be recognized by the detection means, resulting in elevated assay background. Preferably, the enzyme is added at a concentration between 0.05 and 0.5 mg/mil and is incubated for between 10 and 60 minutes. Most preferably, the enzyme is RNAse A (Sigma, St. Louis, Mo.) and is incubated with the captured DNA for approximately 30 minutes at a concentration of 200 μg/ml. RNAse III (NCI, Frederick, Md.) can also be used.

The RNAse and conjugate are preferably diluted in a conjugation buffer that promotes specific antibody-antigen interaction, blocks non-specific binding of conjugate to the capture tube and stabilizes conjugate for long-term storage. A preferred buffer contains 0.1 M Tris™-HCl, pH 7.5, 0.6 M NaCl to reduce cross reaction of antibody with other nucleic acid species, ZnCl₂ and MgCl₂ for stabilizing alkaline phosphatase, normal goat serum to block non-specific interaction of conjugate with the capture surface, 0.25% Tween™-20 to block non-specific binding of conjugate, and sodium azide as a preservative. A preferred wash buffer contains 0.1 M Tris™-HCl, pH 7.5, 0.6 M NaCl, 0.25% Tween™-20, and sodium azide as a preservative.

Detection of the antibody-hybrid conjugate is preferably achieved by binding the above-described labeled anti-hybrid molecule to the hybrid during the RNAse incubation. Tubes are then washed with the above-described wash buffer to remove any excess conjugate. Preferably, five manual washes are performed using either an Eppendorf™ Repeat Pipettor with a 50 ml Combitip™ (Eppendorf, Hamburg, Germany), a Corning™ repeat syringe (Corning, Corning, N.Y.), a simple pump regulated by a variostat, or by gravity flow from a reservoir with attached tubing. Commercially available tube or plate washing systems available from Source Scientific Systems (Garden Grove, Calif.) can also be used.

9. RNA Detection Kit

RNA detection kits for use with the disclosed method can be made using some or all of the components required for the method. Preferably the kit contains at least a reverse transcriptase lacking RNA/DNA hybrid-dependent exonuclease function, and an antibody specific for RNA/DNA hybrids. The kit also preferably contains a primer that is complementary to a region on an RNA molecule of interest, and more preferably contains a plurality of primers that are each complementary to a region on an RNA molecule of interest.

The kit should contain a negative control and a positive control for each probe. Preferably, the negative control is sonicated herring sperm DNA (100 pg/ml) dissolved in sample transport medium. The positive control preferably contains herring sperm DNA (100 pg/ml) and probe target nucleic acid. In general, the assay can be used to detect as little as 1.0 pg to 10 ng of nucleic acid per ml of specimen with a typical specimen volume of 0.1 ml.

10. Direct Sequencing of Target RNA

Target RNA molecules can be isolated and directly sequenced using RNase H⁻ reverse transcriptase with appropriate oligodeoxyribonucleotide primers and chain terminating dideoxynucleotides (Mierendorf & Pfeffer, Methods. Enzymol., 15:563-566 (1987)). The chain terminated hybrids can be treated with RNase A to digest unhybridized RNA portions of the duplex, separated via electrophoresis, blotted on nitrocellulose membrane, and reacted with the anti-hybrid antibody in a fashion similar to a Western Blot. Direct sequencing of the transcripts allows for the analysis for the presence of mutations. Mutational analysis includes but is not limited to, point mutations, deletions and insertions. It will be understood by those skilled in the art that other forms of hybrid separation, immobilization, or solid phase can be utilized in the assay described above.

12. Detection of Reverse Transcriptase Activity and Inhibitors

The disclosed method also allows easy detection of reverse transcriptase activity. This can be useful in the screening and testing for reverse transcriptase inhibitors against organisms harboring this enzyme (such as HIV), even if the reverse transcriptase has an RNase H function. A target RNA can be hybridized to appropriate 5'-biotin labeled oligodeoxyribonucleotide primers, reverse transcribed in the presence of chain terminating dideoxynucleotides and the presumptive enzyme inhibitor. The chain terminated RNA:DNA hybrids are then treated with RNase A (to digest unhybridized RNA portions of the duplex), captured on a streptavidin coated solid phase, and detected with the anti-hybrid antibody. Detection of hybrids is indicative of enzyme activity and lack of inhibition. The amount of hybrid present can also be used to estimate the level of inhibition.

The following non-limiting examples illustrate use of the present assay and kit.

EXAMPLE

The following is an example of a preferred method of performing the disclosed method for the detection of a target RNA in a sample.

Mix a 5:1 ratio (pmol amounts) of 5' biotinylated 20 to 30 nucleotide primers to complementary RNA in a plastic microcentrifuge tube (6 μl final volume). Add 5 μl of hybridization/extension buffer (100 mM Tris-HCl, pH 8.3, 150 mM KCl, 6 mM MgCl₂, 20 mM DTT and 1 mM each dNTP). Anneal the RNA and primer by heating the mixture to the optimal annealing temperature, preferably 60° C. (optimal annealing temperature varies with primer and RNA utilized), for 20-30 minutes. Cool the mixture at 20-27° C. for 10 minutes. Add an additional 5 μl of hybridization/extension buffer and 2 Units of RNase H- reverse transcriptase. Bring up the total final volume to 20 μl by adding Nuclease-free water. Incubate the reaction for 30-60 minutes at 42° C. Add 1 μl of 0.5 M EDTA and 1 μl of 1.0 mg/ml RNase A. Incubate for 30 minutes at 37° C. Transfer the mixture to a streptavidin coated microtiter plate and capture for 30-60 minutes at 20-27° C. with constant shaking (1100 rpm). Add 100 μl of anti-hybrid alkaline phosphatase conjugated antibody mix and incubate at 20-27° C. for 30-60 minutes. Wash the unbound, and add 100 μl of a chemiluminescent substrate. Incubate for 15-30 minutes at 20-27° C. Read the signal utilizing a luminometer.

A form of the above procedure was used to detect HIV-1 RNA in different samples with a known amount of RNA present. A 23 nucleotide biotinylated oligonucleotide was used as the extension primer. The enzymatic reaction used 200 units of SuperScript II RNase H- Reverse Transcriptase (Life Technologies) and the reaction was carried out for two hours at 42° C. The results are shown in Table 2, where the relative light units (RLU) detected in the assays are compared to the number of RNA molecules present and the resulting signal-to-noise ratio (S/N) is calculated.

                  TABLE 2                                                          ______________________________________                                         Molecules    RLU        S/N       S/N-1                                        ______________________________________                                         0            0.0229     1.0       0                                              20,000,000 56.283 2457.8 2456.8                                                2,000,000 8.134 355.2 354.2                                                    200,000 0.6705 29.3 28.3                                                       20,000 0.1038 4.5 3.5                                                        ______________________________________                                    

The relationship of number of RNA molecules to the signal-to-noise ration is graphed in FIG. 1. The graph shows that over the wide range of RNA molecules present, the relationship between the number of RNA molecules present and the signal detected (represented as the signal-to-noise ratio minus 1) is essentially linear.

Publications cited herein and the material for which they are cited are specifically incorporated by reference.

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims. 

We claim:
 1. A method for detecting RNA comprising reverse transcribing the RNA with a reverse transcriptase lacking RNA/DNA hybrid-dependent exonuclease function to form an RNA/DNA hybrid, and detecting the RNA/DNA hybrid with an antibody specific for RNA/DNA hybrids.
 2. The method of claim 1 wherein the method detects the presence of an RNA molecule of interest in a sample.
 3. The method of claim 2 wherein the method comprises(a) mixing a primer and the reverse transcriptase lacking RNA/DNA hybrid-dependent exonuclease function with the sample, wherein the primer is complementary to a region of the RNA molecule of interest, (b) incubating under conditions that promote reverse transcription, wherein the RNA/DNA hybrid is formed if the RNA molecule of interest is present in the sample, and wherein the RNA/DNA hybrid is a hybrid of the RNA molecule of interest and a reverse transcribed DNA molecule complementary to the RNA molecule of interest, (c) mixing an antibody specific for RNA/DNA hybrids with the sample, wherein the antibody binds to the RNA/DNA hybrid to form an antibody-RNA/DNA hybrid conjugate, and (d) detecting the antibody-RNA/DNA hybrid conjugate, wherein the presence of the antibody-RNA/DNA hybrid conjugate indicates the presence of the RNA molecule of interest in the sample.
 4. The method of claim 3 wherein the primer is unique to the RNA molecule of interest.
 5. The method of claim 3 wherein steps (a) and (b) are performed simultaneously.
 6. A method for detecting an RNA molecule of interest in a sample, the method comprising(a) mixing a primer and a reverse transcriptase lacking RNA/DNA hybrid-dependent exonuclease function with a sample, wherein the primer is complementary to a region of the RNA molecule of interest, (b) incubating under conditions that promote reverse transcription, wherein an RNA/DNA hybrid is formed if the RNA molecule of interest is present in the sample, and wherein the RNA/DNA hybrid is a hybrid of the RNA molecule of interest and a reverse transcribed DNA molecule complementary to the RNA molecule of interest, (c) mixing single-strand-dependent ribonuclease with the sample, wherein single-stranded RNA present in the sample is substantially degraded, (d) mixing an antibody specific for RNA/DNA hybrids with the sample, wherein the antibody binds to the RNA/DNA hybrid to form an antibody-RNA/DNA hybrid conjugate, and (e) detecting the antibody-RNA/DNA hybrid conjugate, wherein the presence of the antibody-RNA/DNA hybrid conjugate indicates the presence of the RNA molecule of interest in the sample.
 7. The method of claim 6 wherein the single-strand-dependent ribonuclease is RNAse A.
 8. The method of claim 3 wherein the primer comprises a capture tag, and wherein the method further comprises capturing the primer.
 9. The method of claim 8 wherein the primer is captured on a substrate.
 10. The method of claim 9 wherein the substrate is selected from the group consisting of plates, slides, dishes, beads, particles, cups, strands, chips and strips.
 11. The method of claim 10 wherein the substrate is selected from the group consisting of plates and magnetic beads.
 12. The method of claim 9 wherein the capture tag is biotin and wherein the substrate is coated with streptavidin.
 13. The method of claim 8 wherein the capture tag is biotin.
 14. The method of claim 8 wherein the primer is captured following step (a).
 15. The method of claim 14 wherein the primer is captured prior to step (b).
 16. The method of claim 14 wherein the primer is captured following step (b).
 17. The method of claim 3 wherein the reverse transcriptase is thermophilic.
 18. The method of claim 17 wherein conditions that promote reverse transcription comprise a temperature from 20° C. below the melting temperature of a hybrid between the RNA molecule of interest and the primer to the melting temperature of a hybrid between the RNA molecule of interest and the primer.
 19. The method of claim 18 wherein conditions that promote reverse transcription comprise a temperature from 5° C. below the melting temperature of a hybrid between the RNA molecule of interest and the primer to the melting temperature of a hybrid between the RNA molecule of interest and the primer.
 20. The method of claim 2 wherein the method detects the presence of a plurality of RNA molecules of interest in a sample.
 21. The method of claim 2 wherein the sample is obtained from an animal and wherein the presence of the RNA molecule of interest in the sample is indicative of an infection, a disease state, or predisposition to a disease in the animal.
 22. The method of claim 21 wherein the method detects the presence of a plurality of RNA molecules of interest in a sample.
 23. The method of claim 22 wherein detection of any one of the plurality of RNA molecules is indicative of an infection, a disease state, or predisposition to a disease in the animal.
 24. A kit for detecting the presence of RNA sequences comprising(a) reverse transcriptase lacking RNA/DNA hybrid-dependent exonuclease function, and (b) an antibody specific for RNA/DNA hybrids.
 25. The kit of claim 24 further comprising a primer that is complementary to a region on an RNA molecule of interest.
 26. The kit of claim 24 further comprising a plurality of primers that are each complementary to a region on an RNA molecule of interest.
 27. The kit of claim 24 wherein the enzyme and detecting means are in the same reagent.
 28. The method of claim 3 further comprising labelling the antibody wherein the label is selected from the group comprising capture tags, fluorescers, radionuclides and enzymes and wherein the method further comprises detecting the antibody.
 29. The method of claim 3 further comprising a secondary antibody for specifically binding to the antibody, wherein the secondary antibody is labelled, the label being selected from the group comprising capture tags, fluorescers, radionuclides and enzymes and wherein the method further comprises detecting the secondary antibody.
 30. A method for sequencing RNA comprising reverse transcribing the RNA with a reverse transcriptase lacking RNA/DNA hybrid-dependent exonuclease function in the presence of chain terminating nucleotides to form chain terminated RNA/DNA hybrids, and detecting the chain terminated RNA/DNA hybrids with an antibody specific for RNA/DNA hybrids.
 31. The method of claim 30 wherein the method comprises(a) mixing a primer, the chain terminating nucleotides, and the reverse transcriptase lacking RNA/DNA hybrid-dependent exonuclease function with the sample, wherein the primer is complementary to a region of the RNA molecule of interest, (b) incubating under conditions that promote reverse transcription to form the chain terminated RNA/DNA hybrids, (c) mixing an antibody specific for RNA/DNA hybrids with the sample, wherein the antibody binds to the RNA/DNA hybrids to form an antibody-RNA/DNA hybrid conjugate, and (d) detecting the antibody-RNA/DNA hybrid conjugate, and determining the sequence of the RNA molecule.
 32. A method for identifying inhibitors of reverse transcriptase, the method comprising incubating, in the presence and in the absence of a test compound, a reverse transcriptase lacking RNA/DNA hybrid-dependent exonuclease function, an RNA molecule, and a primer under conditions that promote reverse transcription, and detecting any resulting RNA/DNA hybrid with an antibody specific for RNA/DNA hybrids, wherein if the amount of RNA/DNA hybrid detected in the presence of the test compound is less than the amount of RNA/DNA hybrid detected in the absence of the test compound then the test compound inhibits the reverse transcriptase.
 33. A method for detecting reverse transcriptase activity in a sample, the method comprising incubating the sample in the presence of an RNA molecule and a primer under conditions that promote reverse transcription, and detecting any resulting RNA/DNA hybrid with an antibody specific for RNA/DNA hybrids.
 34. The method of claim 33 wherein the method comprises(a) mixing the primer and the RNA molecule with the sample, wherein the primer is complementary to a region of the RNA molecule, (b) incubating under conditions that promote reverse transcription, wherein the RNA/DNA hybrid is formed if there is reverse transcriptase activity in the sample, and wherein the RNA/DNA hybrid is a hybrid of the RNA molecule and a reverse transcribed DNA molecule complementary to the RNA molecule, (c) mixing an antibody specific for RNA/DNA hybrids with the sample, wherein the antibody binds to the RNA/DNA hybrid to form an antibody-RNA/DNA hybrid conjugate, and (d) detecting the antibody-RNA/DNA hybrid conjugate, wherein the presence of the antibody-RNA/DNA hybrid conjugate indicates the presence of the reverse transcriptase activity in the sample.
 35. The method of claim 33 further comprising incubating the sample in the presence of a test compound, wherein if the amount of RNA/DNA hybrid detected in the presence of the test compound is less than the amount of RNA/DNA hybrid detected in the absence of the test compound then the test compound inhibits the reverse transcriptase activity. 